Mast-type cranes for which the safety brake of this invention are particularly suitable are commonly used in warehouses and are often computer controlled. Such a crane may comprise an undercarriage that is mounted on flanged wheels to ride on a system of tracks extending throughout the warehouse or other operating site. Under the control of a human operator or a computer, the crane can move along the tracks to a selected aisle, thence along the aisle to a selected tier of shelves or drawers in front of which it stops. Projecting up from the undercarriage is a mast along which a carriage or platform moves up or down, to be brought to a predetermined level at which the carriage is opposite a selected one of the shelves or drawers in the tier at which the crane has stopped. Shuttle mechanism on the carriage withdraws the selected drawer from its storage position, and the crane then moves back along the tracks to an unloading station at which a needed item or items can be removed from the withdrawn drawer. The crane can then be sent back with the drawer to return it to its storage position.
The carriage is raised and lowered by means of a tension member--a chain or a wire rope--that is connected to the carriage and is ordinarily trained over a sheave at the top of the mast and thence down to a motor driven winch mounted on the undercarriage behind the mast. Failure of the tension member in a mast-type crane is not a common occurrence, but it is possible for the tension member to break, or for its connection with the carriage to break, and reasonable prudence should make provision for such failure. Accordingly, a well designed mast-type crane is equipped with a safety brake that engages automatically upon failure of the tension member, to hold the carriage against descent. Without a safety brake, the dropping of the carriage that would result from a tension member failure could result in injury to persons or damage to property in the vicinity of the crane, or in damage to the crane itself or to fragile and valuable cargo carried by the falling carriage.
A safety brake for a mast crane should of course operate reliably upon failure of the tension member but should not interfere with normal controlled descent of the carriage. An important consideration is that the safety brake mechanism be compact, so that the aisles along which the crane moves can be narrow for efficient utilization of floor space. The safety brake mechanism must also be vertically compact so that it does not limit the range of up and down motion of the carriage. Inasmuch as the safety brake is in the nature of emergency equipment which, under optimum conditions, is never used, it represents, in one sense, an unproductive investment and therefore low cost is a very important consideration.
A prior safety brake mechanism for a mast-type crane is disclosed by U.S. Pat. No. 3,250,399, to M. J. Dechantsreiter. In that mechanism, the tension member was connected to a generally horizontally extending arm that had one end pivoted to the carriage and had its opposite end normally engaged against a downwardly facing abutment on the carriage, the arm being held up against that abutment by the reaction of the tension member to the weight of the carriage. A normally open microswitch on the arm also engaged the abutment, to be held closed by such engagement and thereby provide for energization of a solenoid on the carriage. Upon failure of the tension member, the arm dropped out of engagement with the abutment, opening the microswitch and thus de-energizing the solenoid. With the solenoid de-energized, the weight of its plunger was eccentrically imposed upon a cam on the carriage to rotate the latter into wedge-like engagement with the mast.
The mechanism just described had the advantages of simplicity, compactness and reliability, but it had the significant disadvantage of requiring electrical supply connections for the switch and solenoid on the carriage that had to accommodate the up and down motion of the carriage. Furthermore, the rotatable cam that comprised the braking element engaged the rail with its periphery, making essentially only line contact with the rail and thus providing only a relatively small braking surface that was not well suited for supporting a heavy load. The brake was self-energizing, but because of the line contact between each cam and the mast, braking security was achieved by having the cam virtually bite into the mast, with the possibility of the guiding surfaces of the mast being scarred or dented if the safety brake was applied with a heavy load on the carriage.
A more recent safety brake that has been used on smaller mast-type cranes has comprised a pair of jaw members located at opposite sides of a braking rail on the mast. One of the jaw members had a braking surface that was parallel to its opposing surface of the rail, the other had a surface that was inclined relative to the rail, extending obliquely upwardly and towards its adjacent surface of the rail. A tapered braking member was confined between the inclined jaw member surface and the opposing rail surface, and upon failure of the tension member the tapered braking member was urged upwardly to wedge itself between the rail and the inclined jaw member. The actuating mechanism for this safety device was relatively complicated and expensive. It needed four different adjustments during assembly and rigging, and it required the provision of a separate rail on the mast that was engaged only by the safety brake elements. Another important objection was that the braking force applied to the rail was dependent upon the wedging action of the tapered member and could increase only if the tapered member slid along the inclined jaw member, whereas the tapered member was just as likely to slide along the rail and be held by friction against sliding relative to the jaw member.